The present invention relates to a method of driving an engine valve-driving electromagnetic valve that drives an intake or exhaust valve of an engine to open or close by electromagnetic force produced by a combination of an electromagnet and a permanent magnet.
It is known that an engine valve is driven by an electromagnetic valve in place of a cam drive mechanism. FIG. 6 is a sectional side view showing the whole structure of a conventional engine valve-driving electromagnetic valve [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. 7-83012]. A port 45 is formed in an intake/exhaust passage (an intake passage or an exhaust passage) 12 of a cylinder head 10 of an engine. A valve head 14 of an intake/exhaust valve (an intake valve or an exhaust valve) is provided so as to be capable of reciprocating toward the port 45, thereby forming an engine valve 11. An electromagnetic valve 1 is provided adjacently to the cylinder head 10. The electromagnetic valve 1 has a casing 2 made of a non-magnetic material. A first core 3 and a second core 4 are provided in the upper and lower end portions of the casing 2. The first and second cores 3 and 4 are annular cores having a U-shaped cross-sectional configuration and made of a magnetic material. A movable element 7 is placed between the first core 3 and the second core 4. The movable element 7 is formed from a magnetically attracting iron plate. The movable element 7 is secured to the distal end of a valve stem 16 of the engine valve 11. A first exciting coil 5 is incorporated in a groove of the first core 3. Similarly, a second exciting coil 6 is incorporated in a groove of the second core 4. When supplied with a predetermined electric current from a driver circuit, the first exciting coil 5 and the second exciting coil 6 each produce a magnetic field whose intensity corresponds to the value of the supplied electric current, causing a magnetic flux corresponding to the magnetic field intensity to pass through each of the first and second cores 3 and 4.
The magnetic flux produced by each of the first and second exciting coils 5 and 6 returns through the first core 3 or the second core 4, the movable element 7 and an air gap lying therebetween. The air gap forms a part of the magnetic circuit. The magnetic reluctance of each of the first core 3, the second core 4 and the movable element 7, which are each made of a magnetic material, is at a level that may be ignored in comparison to the magnetic reluctance of the air gap. The magnetic reluctance of the air gap is a function of the gap length. The smaller the gap, the smaller the magnetic reluctance, and the more stable is the magnetic circuit. When an electric current is supplied alternately to the first exciting coil 5 and the second exciting coil 6 from the driver circuit, an electromagnetic attraction force corresponding to the supplied electric current is produced. Consequently, the movable element 7 is attracted to the first exciting coil 5 or the second exciting coil 6 alternately. Thus, driving force required to drive the engine valve 11 can be obtained.
In the electromagnetic valve 1, if the movable element 7 is driven simply by switching the supply of the electric current, which is supplied to the first exciting coil 5 and the second exciting coil 6 alternately, without giving any special consideration to the mechanism, there will be considerable variations in the time required for the movable element 7 to complete its movement from the instant when the supply of the electric current is switched. Therefore, practical control cannot be realized. For this reason, a vibration system is formed by using springs so that a movable system including the valve body (comprising the valve head 14 and the valve stem 16) and the movable element 7 is held in a predetermined neutral position and the movable system is allowed to vibrate by predetermined free vibration. With this arrangement, when the supply of the electric current is cut off in a state where the movable element 7 is placed in contact with the first exciting coil 5 or the second exciting coil 6, the movable element 7 immediately begins simple harmonic motion away from the exciting coil. Accordingly, the open-close cycle of the valve body can be controlled with high accuracy by controlling the length of time that the movable element 7 is held in contact with the first exciting coil 5 or the second exciting coil 6.
A spring 8 is fitted between-the upper side of the movable element 7 and the upper end of the casing 2. A spring 9 is fitted between the lower side of the movable element 7 and the lower end of the casing 2. The springs 8 and 9 are non-linear springs each having a reduced diameter at a central portion thereof. The spring constant of the springs 8 and 9 is small in a region where the displacement is small, but large in a region where the displacement is large. Accordingly, in the vicinity of the intermediate position, the spring force produced from the springs 8 and 9 is small in comparison to linear springs. Therefore, the arrangement using the springs 8 and 9 is useful for driving. In the vicinities of the open and closed positions of the engine valve 11, the springs 8 and 9 produce large spring force. Therefore, the arrangement does not degrade the response of the electromagnetic valve 1. Thus, the electromagnetic attraction force acting on the movable element 7 and the spring force of the springs 8 and 9 are matched to each other at all times, and the generation of excessive electromagnetic attraction force is avoided.